Not Applicable.
The present invention relates to the production of carbon and/or graphite fibers. More particularly, the present invention relates to a method for carbonizing and/or graphitizing carbon fiber precursors using plasma technology and electromagnetic irradiation.
Carbon and graphite fibers are commonly used as reinforcement materials in advanced structural composites. Advanced structural composites are generally lightweight and possess superior strength and elasticity over most metals. Because of these characteristics, highly advanced composites are now regularly utilized as structural members in the aerospace industry and in high-tech space applications. The use of these composites in other commercial industries, however, has seen limited application due to the high material costs associated with carbon and graphite fibers and the lack of rapid and efficient techniques for their manufacture. Currently, only moderate-cost fibers have found common application in broad consumer markets. These markets typically include the construction of items such as tennis rackets, fishing poles, and golf clubs.
Carbon and graphite fibers are produced through the controlled pyrolysis of fibrous organic carbon precursors such as polyacrylonitrile (PAN), pitch (petroleum or coal tar), or rayon. Generally, rayon-based precursors are used to produce low modulus carbon fibers (fibers having a modulus xe2x89xa650 GPa, or 7xc3x97106 PSI) while PAN or liquid crystalline (mesophase) pitch precursors are used to make the higher modulus carbon fibers (fibers having a modulus xe2x89xa7200 GPa, or 7xc3x97106 PSI) used in advanced composites. Of these precursors, the PAN precursor is generally preferred due to its high carbon yield and unique mechanical properties which intrinsically avoid the need for an expensive final xe2x80x9cgraphitizationxe2x80x9d step.
The process for manufacturing carbon and graphite fibers is generally a lengthy and expensive process. The conventional process begins by spinning the carbon precursor into a fiber form using any one of several different spinning techniques. Once set in fiber form, the carbon fiber precursor is typically subjected to a stabilization step wherein the fiber is heat-treated in air and at relatively low temperatures (approximately 200xc2x0 C. to 350xc2x0 C. or higher). As a result of this stabilization step the outer layers or regions of the fiber are converted to an infusible and thermally stable structure capable of withstanding the high processing temperatures necessary for carbonization to a carbon or graphite form. Depending upon the stabilization process conditions employed, the stabilization process may also result in the conversion of the entire fiber to a fully stabilized form.
To form carbon or graphite fibers, the stabilized carbon fiber precursor is fired in an inert atmosphere at extremely high temperatures while under tension. Carbon fibers are generally achieved by firing at temperatures between 1000xc2x0 C. and 2000xc2x0 C., while higher modulus carbon fibers (graphite) normally require firing at temperatures in excess of 2500xc2x0 C. The high temperatures cause the initial organic material in the fiber to convert into carbon while the fiber""s noncarbon elements are expelled in the form of volatile gases. This off-gas stream is toxic and includes substantial amounts of HCN, NH3, N2, and H2O with lesser amounts of low molecular-weight nitriles, CO2, CH4, CO and H2. Because of the toxic nature of the off-gas stream, treatment by liquid-phase scrubbing or catalytic combustion is required before venting. Typically, the entire carbon/graphite manufacturing process is performed in multiple and sequential conventional graphite brick-lined furnaces and may require hours to complete.
Following carbonization, the carbon or graphite fiber is usually surface treated to enhance its ability to adhere to a sizing agent and a matrix material, usually a polymeric resin. The matrix serves to bind the fibers together, forming a coherent structure and providing a medium for transferring applied stresses from one fiber to another. The matrix material affects the composites high temperature mechanical properties, transverse strength and moisture resistance, as well as other properties, and is a key factor in toughness, shear strength, and oxidation and radiation resistance. The matrix system also strongly influences the fabrication process and associated parameters for forming intermediate and final products from the composite materials.
Untreated carbon and/or graphite fiber surfaces usually have low surface energies which limit their ability to form strong adhesive bonds with matrix materials. Surface treatments applied to these fibers are able to overcome this limitation by increasing the fiber""s surface activity and surface energy. These treatments typically include surface modification processes such as anodic oxidation, electrodeposition, wet and dry oxidation, plasma etchings, coatings, ion implantations, and more. Of these processes, low pressure plasma processing has offered a very attractive and efficient method for modifying the fiber""s surface activity without affecting its bulk properties.
Plasma surface treatment of fully processed (fully carbonized) fibers is a well known technology previously discussed at length by J. C. M. Peng et al., xe2x80x9cSurface Treatment of Carbon Fibers,xe2x80x9d Carbon Fibers, Third Edition, 180-187 (J. B. Donnet et al., ed., 1998); L. H. Peebles, xe2x80x9cPlasma Treatment,xe2x80x9d Carbon Fibers Formation, Structure, and Properties, 128-135 (1995); and J. Delmonte, xe2x80x9cSurface Treatment of Carbon/Graphic Fibers,xe2x80x9d Technology of Carbon and Graphite Fiber Components, 189-191 (1981), incorporated herein by reference. In the typical plasma treatment, the surface concentration of polar (oxygen-containing) groups on the filly carbonized or filly or partially graphitized fiber surface are increased by exposure to an oxygen-plasma treatment. The increased polarity, in turn, leads to both higher epoxy adhesive wetability and stronger intrinsic adhesion across the adhesive/composite interface. Under normal processing conditions, the plasma surface treatment results in extensive modifications to the outmost few atomic layers of the substrate while leaving the bulk properties of the fiber intact.
Currently, over 30,000 tons of carbon fibers are produced annually throughout the world. Although this number may seem substantial, the commercial industry has yet to realize the potential widespread use of carbon fibers because of the high costs associated with their production as compared to other materials. The most significant cost factors include the high cost of carbon precursors (45-50% of production costs), the high cost of equipment and energy consumption (20-25% of production costs), and the time expense associated with producing a quality product. In regards to the latter factor, attempts to speed the process has often resulted in the rapid burn off of the noncarbon elements which, in turn, creates bubbles and cracks in the fiber. These bubbles and cracks substantially weaken the fiber""s mechanical properties such that the fibers are rendered incapable of use for their desired purpose.
U.S. Pat. No. 4,197,282, discloses a technology which is intended to reduce the costs associated with producing carbon fibers from natural organic materials, such as petroleum distillation residues or coal. In this process, carbonized and/or graphitized fibers are manufactured from natural organic precursors using a preparatory thermal treatment step and microwave irradiation. In its application, the natural organic material is spun into a fibrous carbon precursor and then heat treated in an inert atmosphere at a temperature between 300xc2x0 C. and 1500xc2x0 C. in a conventional furnace. The preparatory thermal treatment produces an initial carbonization which allows an interaction between the microwaves and the fibers. As with the conventional process, the inert atmosphere is obtained by using a gas which does not react with the fibers and is resistant to the temperatures reached, e.g., nitrogen, argon, helium or hydrogen. This process may also include giving the fibers an initial oxidation stabilization treatment at a temperature between 100xc2x0 C. and 250xc2x0 C.
After the preparatory thermal treatment, the fibers are subjected to carbonization treatment by irradiation with microwaves. The irradiation by microwaves may be carried out immediately or else discontinuously by storing the thermally pre-treated fibers and then irradiating them later. The irradiation by microwaves is carried out by electromagnetic radiation whose frequency is between 900 MHz and 30,000 MHz and preferably between 2000 MHz and 15,000 MHz, and with a power between 50 W and 10 kW. The fibers obtained by this process are said to have breaking strengths of between 1,000 and 10,000 kgf/cm2.
Unfortunately, this technology has its limitations. First and foremost is the fact that the technology is limited solely to the use of natural organic raw materials and is not effective in producing other synthetic carbon fibers, such as PAN-based fibers. Moreover, the process requires a thermal pre-treatment step wherein the fibers are heated by conventional means to a temperature near the precursor""s carbonization point. This pre-treatment step is necessary to increase the inherently poor coupling efficiency to the electromagnetic field. Although this abbreviated temperature treatment does not utilize the high temperature range as seen in conventional carbonization processes, the general use of a conventional furnace and the subsequent transition to a microwave field is typically inefficient and maintains the high costs associated with conventional processes.
In the absence of new and more efficient processes for manufacturing carbon fiber-based composites, the benefits associated with their use will go unrealized in other commercial industries. For example, within the domestic automobile industry there lies a growing interest in developing carbon fiber-based composites for use in primary structural applications. The use of advanced composites, if practical, would provide significant weight savings in manufactured vehicles, thereby increasing the vehicle""s fuel efficiency, while maintaining the high strength and high modulus mechanical properties necessary for consumer protection.
It is a principle object of the present invention to provide a novel method for manufacturing carbon and/or graphite fibers that will reduce the costs associated with their manufacture and allow widespread commercial use of advanced composites reinforced with carbon or graphite fibers.
It is another object of the present invention to provide a method for producing carbon and/or graphite fibers that does not require carbonization by heat treatment in high temperature furnaces.
It is yet another object of the present invention to provide a method for utilizing plasma technology and electromagnetic radiation to manufacture carbon and/or graphite fibers.
These and other objects are achieved by the present invention. The present invention is summarized in that it provides a novel method for producing carbon and/or graphite fibers utilizing plasma technology and electromagnetic radiation. The present invention also includes carbon and/or graphite fibers so obtained. Specifically, the present invention discloses a method for producing carbon and/or graphite fibers wherein stabilized carbon fiber precursors are placed in an oxygen-free atmosphere, under slight tension and/or physically restrained, and subjected to both a plasma energy and a level of electromagnetic radiation which is increased as the fibers progress towards a final carbon or graphite product. The plasma is generated in a controlled oxygen free plasma chamber using an oxygen free inert gas capable of acting as a carrier for the generated effluents of the processing system. The electromagnetic radiation is generated by a standard electromagnetic generator capable of providing electromagnetic radiation within the microwave frequency range and a power input between 250 W and 100 kW.
In its practice, the stabilized carbon fiber precursors are physically restrained and/or placed under slight tension and subjected to the plasma in the plasma chamber. The plasma in the plasma chamber interacts with the stabilized carbon fiber precursor and initiates the pyrolysis process in the fiber while increasing the fiber""s dielectric loss tangent and consequently raising the fiber""s coupling efficiency to the electromagnetic radiation. Through this coupling, a uniform application of electromagnetic energy is achieved throughout the fiber""s cross-section, the uniform application of electromagnetic energy results in uniform and homogeneous volumetric heating which promotes the mass exchange of oxygen and evolved gases across the entire cross-section of the fiber. These gases are released in the form of off-gases and serve as an indicator for increasing the level of electromagnetic energy or the completion of carbonization. In an additional step, oxygen may be carefully introduced into the chamber after the fibers are carbonized or fully or partially graphitized to surface treat the carbon or graphite product to assist in matrix adhesion.
It is an advantage of the present invention that carbon and/or graphite fibers are capable of being manufactured according to the disclosed method from a wide range of carbon fiber precursors. In addition, the present method is capable of producing carbon and graphite fibers having a wide range of final properties as a function of the processing parameters utilized.
It is another advantage of the present invention that carbon and graphite fibers are capable of being produced without requiring additional heat-treatment steps beyond stabilization as required in other carbonization or graphitization processes.
It is still another advantage of the present invention that off-gases produced during the carbonization process are further used to supplement the plasma reaction and drive carbonization, thus reducing the amount of volatile gases present in the effluent gas stream and the amount of volatile gases requiring purification prior to venting.
It is still yet another advantage of the present invention that the disclosed method allows the production of surface treated carbonized or graphitized products in a single process step.
It is yet another advantage of the present invention that PAN-based fibers produced according to the disclosed method have a modulus of elasticity near half that of steel, a strength fifty percent greater than common structural steel, and a density that is near twenty percent that of steel.